Who is in the wrong?

Every year around the holidays, my mother and father get into the worst arguments over who Christmas day should be spent with. My dad’s mother (my grandmother) insits that my dad spend Christmas day with her even though she knows my mom cooks dinner every year on Christmas day. She belittles my father and acts like he never does anything for her. He helps pay her bills, drives her places, visits her at least three times a week etc. She is 70 years old but she is in perfect health. She tells my father that he should put her first and it’s been this way for as long as I can remember. Help?